1. Field of Invention
The invention relates to switching power supply sources in general and particularly switching power supply sources with an inductance for which the voltage level is defined by two control switches.
2. Discussion of Related Art
FIG. 1 illustrates a switching power supply source 1 according to the state of the art. This switching power supply 1 is a DC-DC step up voltage converter like that described in document FR-2 841 061. The switching power supply source 1 has an inductance L connected firstly to a DC power supply source Vbat and secondly to an intermediate node Lx. The source of an NMos transistor N1 is connected to the ground and its drain is connected to the node Lx. The drain of a PMos transistor P1 is connected to the node Lx and its source is connected to the output Vs from the switching power supply 1. A load R to be powered and a capacitor C are connected in parallel between the output Vs and the ground. A switching power supply control device 2 is connected to the gates of the transistors P1 and N1. The control device 2 controls transistor P1 and N1 such that one is blocked (non-conducting) when the other is conducting, so as to prevent current passing through the inductance becoming negative. To prevent transistors P1 and N1 from conducting simultaneously when one transistor is blocked, the other transistor is held blocked for a period usually between 20 and 100 ns before it is made conducting.
During the transient phase in which the two transistors are blocked, the energy stored in the inductance is dissipated in the substrate through the parasite PNP transistor intrinsically formed by transistor P1 when using CMOS technology. This current dissipation into the substrate reduces the energy efficiency of the switching power supply. Furthermore, this current can disturb operation of other devices integrated into the substrate. Moreover, when the parasite PNP transistor formed by P1 is conducting, the intermediate node Lx experiences an overvoltage that can reduce the reliability of the transistor N1. The invention is intended to solve one or several of these disadvantages.